The present invention relates to a video signal processing apparatus which processes plural kinds of video signals and is used when each processed video signal is displayed on the same display device. In particular, it relates to a subtitle processing circuit for magnifying a video signal with a letter box format which displays a horizontally long picture on a screen with a 4:3 aspect ratio into a video signal corresponding to a horizontal long screen having an aspect ratio such as 2:1 or 16:9 and a separation circuit for a helper signal such as a horizontal high frequency band helper signal used in a television system, such as an EDTV 2 system having a helper signal for picture quality improvement.
Recently, besides television systems such as an NTSC system and a PAL system which are now practical and used worldwide, various kinds of television systems which are compatible with the current television systems and have improved resolution and S/N ratios, such as the EDTV (Enhanced Definition TeleVision) 1 system, the EDTV 2 system which is a second generation system of the EDTV system, the IDTV (Improved Definition TeleVision) system, the Improved PAL, the Extended PAL, etc., have been practically used or experimented. Because these new television systems are fundamentally compatible with a current television system, television receivers have to be able to receive signals of plural kinds of television systems.
An example of a television system which has no compatibility with the current above-listed television systems and has high performance is the HDTV (High Definition TeleVision) system. However, receivers for the HDTV system are presumed to be in a transient existence as single system receivers or to function to receive a signal of a current television system besides an HDTV system.
Thus, it will be desired more and more in the future that pictures from various different types of television systems are displayed on the same screen.
It is necessary to convert video signals for various different kinds of television systems into a video signal which can be displayed in common on a screen with an aspect ratio based on one of the television systems. One of the problems, in this case, is the signal conversion of television systems with different aspect ratios.
In the case in which a video signal with a 16:9 aspect ratio is displayed on a screen with a 4:3 aspect ratio, for example, a video signal is often converted so that non-picture portions are generated at the upper and lower portions of the raster (this format is called "letter box") as shown in FIG. 5(a) and the non-picture portions are used for superimposing a subtitle. When a video signal with such a letter box format is displayed on a horizontally long screen with a 16:9 aspect ratio as shown in FIG. 5(b), if the raster is magnified so that the picture portion fills the screen, the subtitle may be off the screen. Therefore, a subtitle processing circuit which moves the subtitle position as shown in FIG. 5(b) by storing subtitle information in a memory is proposed, for example, in Japanese Patent Laid-Open 2-179080.
In this case, although the memory has to store all the video signal including the subtitle information, because the subtitle is located on the non-picture portion at the lower half of the picture, it is necessary to store a maximum of 910 pixels at 4 fsc (fourfolds of subcarrier frequency) for about 130 lines at the lower half portion of the picture. That is,
910 (number of pixels per line).times.130 (number of lines).times.8 (number or gradation)=946,400 bits=about 1 M bits is required as memory capacity.
Usually in television systems such as the EDTV 2 and the Extended PAL systems, a helper signal which is necessary to improve the resolution is usually multiplexed and is demodulated at a receiver side and is used for improving the resolution. For example, in an EDTV 2 system, a horizontal high frequency band helper signal for improving horizontal resolution, utilizing a frequency gap on the position which is symmetrical to a chrominance signal band, is multiplexed on a video signal and transmitted.
It is generally necessary to use a memory to separate such a helper signal at the receiver side. It is also necessary to use a memory to separate a horizontal high frequency band helper signal of an EDTV 2 system at the receiver side. For example, a circuit of this type is shown in FIGS. 4 and 14 on page 137 of "Multidimensional Signal Processing of Television Image" by Toshihiko Fukinuke, published by Daily Industry Press Co. In this case, the memory has to store all the video signal as one field color signal. Therefore,
455 (number of pixels per line).times.262.5 (number of lines).times.8 (number of gradations)=955,500 bits=about 1 M bits is required as memory capacity.
Therefore, two memories, a memory for a subtitle processing circuit and a memory for a horizontal high frequency band helper signal separation circuit, are required.